Since Toyota's pioneering work on polymer layered silicate nanocomposites, a great deal of research has been carried out in this field over the past decade. With the addition of a very small amount of nanofiller into the polymer matrix, these nanocomposites exhibit substantial increase in many physical properties, including mechanical properties (tensile modulus and strength, flexural modulus, and strength), thermal stability, flame retardance, and barrier resistance. Smectite clays, such as montmorillonite (MMT), are of particular interest because they offer a high aspect ratio (10–1000) and a high surface area. Montmorillonite is hydrophilic in nature, which hinders the homogeneous dispersion in the organic polymer phase. Ion exchange of the interlayer inorganic cations (Na+, Ca2+) with organic cation renders the hydrophilic clay surface organophilic. The reduction in surface energy improves the wetting characteristics of the clay surface with polymers or monomers.
From the structural point of view, two idealized polymer/clay nanocomposites are possible: intercalated and exfoliated. Intercalation results from the penetration of polymer chains into the interlayer region and interlayer expansion. Usually the ordered layer structure is preserved and can be detected by X-ray diffraction (XRD). By contrast, exfoliation involves extensive polymer penetration and silicate crystallites delamination, and the individual nanometer-thick silicate platelets are randomly dispersed in the polymer matrix. Exfoliated nanocomposites; usually provide the best property enhancement due to the large aspect ratio and surface area of the clay.
Melt intercalation and in-situ polymerization are the two most common ways of preparing polymer/clay nanocomposites. Melt intercalation involves the mixing of clay with a molten polymer matrix. The layer separation, especially exfoliation, depends on the establishment of favorable interactions between the polymer and the clay surface and the subsequent system energy reduction. In-situ polymerization involves monomer intercalation followed by polymerization. By tailoring the interaction between the monomer, the surfactant, and the clay surface, the exfoliated nanocomposites (nylon-6,2 poly(ε-caprolactone), and epoxy) were successfully synthesized via ring-opening polymerization. The functional group in the organic cation can catalyze intralayer polymerization and facilitate layer separation.
Many thermoplastic polymers are produced by free radical polymerization using vinyl monomers, and the reaction adopts distinctly different mechanisms as compared to ring-opening polymerization. Polymerization is usually carried out in the presence of initiators. It is therefore necessary to investigate the interaction between the monomer, the initiator, and the modified clay surface in a systematic manner. Intercalated polymethyl methacrylate (hereinafter referred to as “PMMA”) and polystyrene (hereinafter referred to as “PS”) nanocomposites have been synthesized through either emulsion or bulk polymerization. It was found that the structural affinity between the styrene monomer and the organic cation played an important role in the PS/clay hybrid structure. Improved dispersion of clay in the PMMA/clay nanocomposites was obtained when methyl methacrylate (hereinafter referred to as “MMA”) was co-intercalated and copolymerized with lauryl methacrylate (LMA). The authors attribute this improvement to a better compatibility of LMA with the organic cation functionalized clay surface. Efforts have also been made to anchor a living free radical polymerization (LFRP) initiator in the interlayer region to improve the intralayer polymerization rate, to achieve an exfoliated PS nanocomposite. Recently, a more conventional initiator, 2,2′-azo(isobutylamidine hydrochloride), was immobilized in the interlayer region to yield exfoliation of clay in the PMMA matrix, using suspension polymerization. Exfoliation of clay in PMMA was also reported in the emulsion polymerization. Our work focuses on the synthesis of exfoliated PMMA and PS/clay nanocomposites via bulk polymerization. The effects of monomer, initiator, and clay surface modification on the structure of nanocomposites are investigated.